In the art of fiber preform manufacture for transmission fibers, techniques have been developed for high speed manufacture using a chemical vapor deposition process, which has been found to be relatively inexpensive, while also providing a high quality fiber. In this process, the necessary cladding and core constituents are supplied in their vapor phase to a horizontally rotated refractory tube to form one or more inner glass layers on the inside surfaces of the tube. Exemplary of this technique is U.S. Pat. No. 4,909,816, issued to MacChesney et al., and its companion patents U.S. Pat. Nos. 4,217,027 and 4,334,903, disclosing what is referred to in the art as the “modified chemical vapor deposition” (MCVD) process.
Conventional MCVD processes of forming optical fiber preforms have been found to result in allowing OH− groups to form in the fiber material. In particular, hydrogen occurring in elemental or compound form is often found in the reactants used for making the preform. Subsequent to formation of the fiber, hydrogen may also become incorporated in the composition and thus compromise the transmission qualities of the fiber. A number of strategies have been developed to avoid this hydrogen incorporation, including the use of hermetically sealed fiber cables, or cables exhibiting various layers of outer cladding to prevent hydrogen intrusion.
As an alternative, a hydrogen gettering process may be used, as disclosed in U.S. Pat. No. 5,596,668 issued to DiGiovanni et al. on Jan. 21, 1997. In the DiGiovanni et al. arrangement a species for gettering or bonding with hydrogen (such as a metal) is included in the cladding layer of the fiber. Diffusion of hydrogen into the fiber is thus trapped in the cladding before it can encroach the core region.
U.S. Pat. No. 4,125,388 issued to Powers on Nov. 14, 1978 discloses a method for making a high purity optical waveguide that exhibits a very low water concentration, where the presence of water (like hydrogen) leads to attenuation in various transmission bands of interest. Powers discloses a method of reducing water attenuation by removing water from the soot preform during the step in which a soot preform is heated to fuse the soot particles into a glass. Powers discloses the use of Cl2 gas as a drying agent. The Cl2 may be fed directly to the preform, or a metal halide gas (such as GeCl4 or SiCl4) may be used together with an oxidizing agent to produce Cl2 in the vicinity of the preform. The drying is carried out within a temperature range in which the soot will fuse into a dense glass.
In an alternative process, a continuous admixture of deuterium with a flow of a carrier gas (such as oxygen) during the preform manufacturing process allows for the isotropic substitution of the deuterium for the hydrogen in the hydroxl group. This substitution results in shifting the absorption peaks out of the wavelength range of interest for optical communication purposes.
A disadvantage of these various techniques is the need for an oxidizing agent, since the presence of excess oxygen in the glass makes the fiber more susceptible to hydrogen-induced loss during subsequent cabling or use in the field. Moreover, the number of defects in the fiber is known to be directly proportional to the induced attenuation upon post-processing exposure to hydrogen.